Crystal oscillators are found in many electrical circuits, and are devices that are fabricated to resonate at predefined frequencies in response to applied voltages. For example, a color burst crystal resonates at a frequency of 3.57954 megahertz (MHz) and may be found in many televisions and radios. Many systems and circuits utilize crystal oscillators to provide a clock reference representative of relative time that is the crystal oscillators provide an oscillation source to a clock generator circuit. A clock generator takes an oscillation source input and generates a digital clock for use in electronic circuits. For example, microprocessors and microcontrollers typically utilize crystal oscillators to derive system clocks that control the rate at which data is read by input/output ports and/or the rate at which programming instructions are executed. Communication systems and components such as telecommunications infrastructure and mobile units use crystal oscillators to generate one or more frequencies that are useful in producing radio frequency (RF) signals onto which information to be broadcast and received is imparted. Crystal oscillators also have tolerance ranges associated with their resonant frequency, and a frequency offset in a crystal oscillator. For example, a crystal oscillator is usually used to generate a master reference clock that is used to synchronize information exchange between telecommunications infrastructure and mobile units.
To control more precisely the resonant frequency of a crystal oscillator, conventional communication systems utilize a digitally controlled crystal oscillator (DCXO) system in conjunction with a crystal. A DCXO system typically includes a processing portion that monitors the resonant frequency produced by a crystal oscillator and alters the resonant frequency of the crystal oscillator by outputting a code to a DCXO circuit that changes the capacitive loading on the crystal oscillator to tune the frequency of the crystal oscillator.
In practice, when a DCXO system is first powered up, for example when a mobile telephone is switched on, an initial DCXO code is used to set the loading capacitance of the crystal oscillator. The initial frequency output by the crystal needs only to be within a few parts-per million (PPM) of the target frequency. After communication is established with another entity, fine frequency tuning may be carried out during which the DCXO changes its output code to refine the load capacitance and bring the resonant frequency within fractions of a PPM of the target frequency i.e., to lower the offset.
Based on various environmental characteristics, such as process, voltage, and temperature (PVT) variations, the resonant frequency of a crystal oscillator may not meet the initial frequency accuracy of a few PPM under all conditions that is an initial offset of the frequency system may be larger than desired. Therefore, many manufacturers calibrate a DCXO with an initial code for operation with a particular crystal oscillator at one specific temperature and store the DCXO code associated with that crystal in memory, such as flash memory, before shipping the product. In the field, when the device attempts to establish initial communication, the DCXO code stored in memory is applied as a first attempt to load the crystal oscillator to achieve the desired oscillator frequency.
However, even when the pre-calibrated DCXO code is loaded, there is no guarantee that the ambient temperature of the crystal oscillator is the same as the calibration temperature at which the initial DCXO code was selected. Additionally, there is no guarantee that the temperature coefficient of the crystal, the supply voltage, and the DCXO circuit will not shift the frequency offset produced using the DCXO code to an unacceptable level. Further, as crystals age, their resonant frequencies may change, thereby potentially rendering the initial DCXO code ineffective. A further disadvantage related to startup of oscillators is the amount of negative resistance applied to the oscillator before oscillations begin. Effects of drive level dependency (DLD) in crystals produces startup problems in oscillators because of the motional resistance of the DLD for crystals at very low drive levels increases rapidly and often abruptly. Yet a further disadvantage is that changes in resistance at lower drive levels and changes in frequency at higher drive levels become noticeable. The high resistance at low drive levels causes startup problems because it is at this point that the oscillator starts from the inherent noise contained in the oscillator's bandwidth and the power levels are orders of magnitude lower than at normal large signal operation. Further, properties like surface micro-cracks and particle contamination also causes this phenomenon.
It would therefore be desirable to have an improved oscillator circuit, the oscillator capable of initiating oscillations with minimum load during startup.